In recent years, group-III nitride such as gallium nitride (GaN) has been attracting an attention as an element that can realize a high-quality short-wavelength light-emitting diode and laser diode. There are many issues to be solved in putting electronic devices and others that use such a group-III nitride structure into practice.
A technique for growing a semiconductor crystal, for example, the epitaxial technique, the MOCVD (Metal Organic Chemical Vapor Deposition) technique, or the like, has controllability in a lamination direction. Typically, however, in fabricating a structure in an in-plane direction, processing must be carried out using a different technique. The crystal processing technique can be roughly classified into a top-down type in which the crystal is processed after the crystal growth and a bottom-up type in which the substrate is processed before the crystal growth, and the structure is fabricated simultaneously with the crystal growth. In the top-down type, the crystal is damaged in the processing and, in particular, in a fine structure, the surface area is large, thereby raising an issue. On the other hand, by the fabrication method of the bottom-up type, there are many cases in which both the controllability of the structure and the crystal quality can be ensured.
With regard to the nitride semiconductor, there is a method for using a mask such as silicon oxide as a fine structure fabrication technique of bottom-up type. This method in which the crystal is grown selectively on the opening part of the mask is a technique that is practically used in the vapor deposition method. However, in the molecular beam epitaxy method (hereafter referred to as MBE), a polycrystal is deposited on the mask.
M. Yoshizawa and others have found out a method for forming a fine columnar gallium nitride crystal having a diameter of about 100 nm in a self-organizing manner by growing gallium nitride under excessive nitrogen in the MBE using active nitrogen excited by high-frequency plasma as a nitrogen source (See the non-patent document 1).    [non-patent document 1] M. Yoshizawa, A. Kikuchi, M. Mori, N. Fujita, and K. Kishino, Jpn. J. Appl. Phys. Vol. 36 (1997), pp. L459-L462